These studies are designed to provide new knowledge about the influence of chronic hypertension on the cerebral circulation. First, the susceptibility of the blood-brain barrier to acute hypertensive disruption appears to be altered in some, but not all types of chronic hypertension. I propose to examine the susceptibility of the blood-brain barrier to acute hypertensive disruption in several models of chronic hypertension to determine whether chronic hypertension has a consistent effect on the susceptibility of the blood-brain barrier to acute hypertensive disruption. I will quantitate the degree of disruption of the blood-brain barrier using two methods, 1) the brain/blood ratio of radiolabelled dextran from an exposed section of the pial microcirculation. Second, recent studies have provided new evidence suggesting that increases in pressue in pial veins is an important determinant of disruption of the blood-brain barrier during acute hypertension. I will study microcirculatory mechanisms of altered susceptibility of the blood-brain barrier to acute hypertensive disruption in rats with chronic hypertension. I will use the new technique of measuring pressure in pial veins of rats to determine the mechanism of acute hypertensive disruption of the blood-brain barrier in rats with chronic hypertension. These studies will provide new information on the mechanisms of the effect of chronic hypertension on the susceptibility of the blood-brain barrier to acute hypertensive disruption.